Study On Reverse Electrodialysis To Recover Energy From Oceanic Salinity Gradient

As the world’s population growing continually, the demand for energy is also in constant increase. Consequently, human are forced to develop and utilize environmentally friendly and renewable energy. Salinity gradient power is the chemical potential energy that exits between sea water and freshwater or two kinds of sea water with different salt concentrations, and the ocean energy in the form of chemical energy. Reverse electrodialysis(RED) is a method using salinity gradient power for power generation,which has higher energy density compared with other methods. The process of the reverse electrodialysis was studied in this paper. The mathematic model of reverse electrodialysis process was established. The effects of several different operational parameters such as flow rates, solution concentrations, channel thickness are investigated.The effects of flow rate, spacer thickness and optimum length on the power density and energy efficiency of the reverse electrodialysis feed with sea water and river water were analyzed with the established model. The result indicate that the change of the flow rate affect the process of the ions across the membrane and the distribution of the solution concentration in the compartment. High velocity of flow keeps the ion concentration on the border of the membrane and solution basically unchanged. The power density and pumping consumption of the RED increase when the flow rate increases. The net power output comes to a maximum value when the flow velocity is 100 mm/s. Reducing the thickness of the compartment can greatly improve both the power density and the energy efficiency.Higher energy efficiency can be obtained as the flow length increases, but the average power density decreases.Concentrated seawater produced by the reverse osmosis desalination process was used for the reverse electrodialysis along with sea water. The effects of fresh water recovery ratio,multistage operation, ion exchange membrane on the generation performance of RED were studied. The results show that using high concentrations of seawater can improve the power output. Multistage feed operation makes the energy efficiency increase from 25.83% to53.57%. Ion exchange membrane with low resistance and high permselectivity is beneficial to improving the high power density output for reverse electrodialysis.